This invention relates to compositions for use with molded brake shoes and in particular to a composition for backer stock material that goes between the wear stock or friction material and the metal backer plate of a brake shoe. In particular the backer stock composition is particularly well adapted for heavy duty service such as encountered by brake shoes in railroad service.
Molded composition brake shoes for railroad service such as the products disclosed in U.S. Pat. No. 2,686,140; 2,681,964; 3,152,099; 3,390,113; 3,832,325; 3,959,194 and 4,178,278 and others have employed friction materials suitable for railroad service which are molded to the metal backer plate of a railroad brake shoe by the use of a backer plate composition which is molded with the friction material against the metal backer plate providing a bond between the backer plate and the friction material.
Backer stock used in this manner with the friction material or wear stock and the metal backer plate has been used in the past to provide a satisfactory bond between the wear stock and the metal backer plate since friction compositions for wear stock are not always easily bonded directly to the metal backer plate in a manner that is entirely suitable for railroad service use. Other backer stock utilized in this application is available which is adhesively applied to the wear stock instead of being molded in place and therefore need not embody the properties required for adhesion during the molding process.
The properties of a backer stock necessary for railroad applications therefore requires that it be capable of adhering strongly to the metal backer plate as well as becoming an integral part of the rear surface of the wear stock nearest to the metal backer plate. It is also important that the backer stock be somewhat flexible or softer than the friction stock. It is also highly desirable that the backer stock be capable of exhibiting sound dampening properties in service. These properties have been achieved in the past by the use of compositions which contained substantial quantities of asbestos fiber. The properties of the prior backer stock material including the superior strength and resistance to heat degredation provided by the asbestos fiber reinforcement has been difficult to obtain using compositions not containing asbestos fiber. It has been desirable, however, to provide compositions which are free of asbestos fiber while still being capable of meeting the requirements for railroad brake shoe service.
In addition, environmental concerns, have imposed an additional criteria that railroad brake shoes in service, be constructed to perform in a manner which would reduce the sound inevitably associated with the application of brake shoe friction materials against the rim of a metal railroad wheel. Finally, economic considerations have required that the foregoing properties and objectives has to be achieved without the use of asbestos reinforcing fibers in a manner which utilizes current molding equipment generally employed for the manufacture of railroad brake shoes in order to provide a product having a manufacturing cost which can make it attractive for use in railroad service.
It is therefore an objective of the present invention to provide a backer stock material which can be incorporated into existing production equipment in the fabrication of brake shoes for heavy duty railroad and AAR (Association of American Railroads) service which is capable of bonding adequately to the friction material used and to the backer plate of a railroad brake shoe by utilizing conventional molding and adhesive techniques, while providing an asbestos-free material which provides superior sound attenuation during use.